Drying kinetics and quality dynamics of ultrasound-assisted dried selenium-enriched germinated black rice

Highlights • Drying efficacy of ultrasonic treatments were test first time on selenized black rice.• Ultrasound-treated samples had greater selenium levels than untreated.• Ultrasonic dried samples significantly decrease drying time of germinated black rice.• The Hii model best fitted the drying kinetics of selenium enriched black rice.• Sonicated samples improved phenolic, anthocyanin, and volatile compound profiles.


Introduction
Black rice originated from China and is mainly cultivated in Southeast Asian countries, and it is the healthiest variety of rice. Black rice is also a functional food abundant in health-promoting flavonoids, polyphenols, essential secondary metabolites, anthocyanins, and antioxidants, which can prevent the development of cancer cells, atherosclerosis, hypertension, diabetes, osteoporosis, asthma, gastrointestinal issues, etc. [1]. Despite its numerous nutritional qualities, black rice is an undervalued rice crop due to its high price and expected market availability [2].
Selenium is a vital human element in biological processes such as antioxidant content and immunological modulation [3]. Inadequate selenium levels can lead to various health issues, such as Kashin-Beck disease and Keshan disease [4]. In regions such as Tibet, where the soil lacks sufficient selenium, the population is greatly affected and has poor health. It is, therefore, essential to find effective methods for increasing selenium levels. Germination is a conventional technique for enhancing the nutritional content of grains. Germination may improve the concentration of folic acid, γ-aminobutyric acid, polyphenols, and other physiologically active components in grain [5]. In addition, germination is an efficient method for selenium accumulating in grain. A previous study found that brown rice can get selenium during germination, primarily in selenium-containing proteins [6]. The study also found that selenium-containing proteins purified from germinated brown rice that had been selenized (Se-GBR) had intense antioxidant activity and could be utilized as effective antioxidants. Numerous other Se-enriched foods, such as rice grain [4], brown rice [6], green tea [7], and Zea mays [8], have acknowledged that Se exerts powerful effects on particular intracellular selenoproteins and potent antioxidant activities. It would be interesting to investigate whether ultrasonic treatment of dried black rice improves its nutritional qualities while maintaining its selenium content, as there is currently no such research.
Drying is an economical and efficient way of preserving grains for lengthy periods. Hot-air drying is the most common drying method, but it is time-consuming. Our previous research has demonstrated that hotair drying may reduce sample quality by causing enzymatic browning and shrinkage and a loss in rehydration and phenolic chemicals [9,10]. In addition, sweet potato microstructure was significantly altered during hot-air drying [11]. To address the limits of conventional drying techniques, enhanced ultrasonic hot-air-drying methods are often utilized to reduce drying time and increase product quality [12,13]. Ultrasonic technology involves using mechanical waves to create ultrasonic cavitation, which leads to increased particle movement and modification of the material's internal structure. This results in improved water diffusion and enhanced dehumidification through heat energy. The impact of high-intensity ultrasound on drying kinetics is predominantly driven by mechanical factors [14]. Fresh fruits and vegetables are commonly dried using ultrasonic technology to maintain product quality. A study conducted by Yang et al. and Zhang et al. [15,16] investigated the effects of ultrasonic treatment on the metabolomics of soybean seeds and peanuts and discovered significant changes in the levels of amino acids, organic acids, and sugars, as well as increased levels of several antioxidants such as glutathione and vitamin C. However, the application of ultrasonic technology in the drying process of germinated black rice seeds is not yet established.
Rice is valued for its flavors, nutrient benefits, and other characteristics. Rice volatiles is one of rice's primary features, significantly influencing rice quality. However, the drying process is rigorous, and rice volatiles may vary over time owing to oxidation and mass loss. Since flavor is a crucial aspect of excellent freshness [17], various research has demonstrated that its chemical content is assumed to be essential for its quality. The method of headspace solid phase microextraction (HS-SPME) is a commonly used approach for analyzing volatile compounds in food and beverage products. This is due to its ease of use, versatility, quick sample preparation, and sensitivity. Additionally, the combination of gas chromatography and mass spectrometry (GC-MS) has significantly reduced both quantitative and qualitative analysis times [6,18]. However, few systematic studies imply SeGBR quality changes after drying or identify SeGBR phenolic compounds and anthocyanins with taste characteristics.
The purpose of the study was to analyze the impact of various drying temperatures (50, 60, and 70 • C) and ultrasonic durations (10, 20, and 50 min) on the drying attributes, mathematical modeling, thermodynamics, energy consumption, microstructure, selenium concentration, bioactivity, and anthocyanin profile of germinated black rice (GBR). The flavor of Se GBR treated with ultrasound was analyzed using HS-SPME and GC-MS. This research may help to give theoretical guidelines for ultrasonic enhanced grain hot air drying.

Raw material collection
Rough black paddy rice with a high germination rate (>85%) of variety Yanghei No. 3, obtained from the Henan Academy of Agricultural Science (China), was used in this study. The black rice was obtained directly from rough rice hulled with a machine. The broken rice was separated by a broken rice separator (FQS-13X20, Taizhou grain instrument Co., Ltd. China).

High-intensity power ultrasound (HIPU)
Sanitization of the samples (360 g) was accomplished before the application of ultrasound by soaking them in 0.1% sodium hypochlorite (500 mL) for 30 min and then washed twice with deionized water. Using an ultrasonic irradiator (Tianhua Co. located in Jining, China), the sterile grains were subjected to ultrasonic stimulation at 40 kHz with an acoustic power of 500 W/cm3 for a holding time of 10, 30, and 50 min.

Germination process
After the ultrasonic treatment, the samples germinated in the dark with 60 μM sodium selenite at 25 • C for 60 h. After germination, the sample was washed with ultrapure water. Samples without ultrasonic treatments and selenite solution were used as the control group. Selenium-enriched black rice was prepared following a described method [19].

Convective hot-air drying (HAD)
The samples were dried in a hot-air dryer with a temperature (of 50, 60, and 70 • C) and 1.5 m/s air velocity. Each 15 g sample was dried until a uniform constant weight was obtained. After a 10-min interval, sample weights were taken until the required moisture content of 5.01.0% was not obtained.

Drying kinetics and mathematical modeling
The experimental result of the study was combined with the 15 drying equations presented in supplementary Table S1. In addition, the following Eqs. (1) and (2) were used to calculate the moisture ratio (MR) and drying rate (DR) during drying experiments.

Non-linear regression analysis
Four statistical functions were used to measure the good fit between the modeling and the experimental data (R 2 , RMSE, RSS, and reduced χ 2 ). A maximized R 2 and minimized RSS, RMSE, and χ 2 values indicate a better model fit.

Activation energy and thermodynamics
where E a is defined as the activation energy (kJ/mol), R g is the universal gas (R g = 8:31451 J/mol/K), and D 0 is an integration constant (m 2 /s).

Total energy consumption calculation (ET HA )
The total energy consumption (ET HA ) in a convection dryer was defined as the total electrical energy applied to determine the blower (E b ) and the electric energy heater (E e ) during the drying process. The energy consumption by the heaters was determined using Eq (7): Energy consumption designed applying (8): 2.9. Quality attributes 2.9.1. Preparation of extract Samples (2.5 g) were sonicated (40 kHz) in an ultrasonic cleaner at 60 • C for 60 min after being dissolved in 5 mL of 68% (v/v) aqueous ethanol. A rotary evaporator operating at 60 • C was used to extract the solvent after the mixture had been centrifuged at 4,000 rpm for 10 min. The extracts were stored at − 25 • C in a brown bottle until further examination.

HPLC analysis of phenolics, flavonoids, and anthocyanins.
HPLC analysis of phenolic acids and flavonoids was performed using a Waters HPLC (model E2695) with a UV-visible photodiode array detector. The HPLC analysis was conducted following the protocol fully described by Peanparkdee et al. [20].

HS-SPME/GC-MS analysis
2.5 g of each sample was placed in 20 mL glass vials with screw-on covers and sealed with septa. The sample (5.00 g) was mixed with 100 μL of the internal standard (3-octanol at a concentration of 50 ng/ mL in distilled water) and 5.00 g of sodium sulfate during the sampling process. Volatile chemicals were extracted and absorbed using SPME fibers (50 μm DVB/CAR/PDMS from Supelco, Pennsylvania, USA) at 60 • C for 30 min. Using an automatic autosampler, the fiber-bearing volatile compounds were immediately injected into the GC inlet and desorbed for 5 min at 250 • C. According to our earlier study by Liu et al., the gradient program and data processing were followed (2016). We used the gradient program for this experiment and processed the data described in our earlier work with Li et al. [6].

Microstructure analysis
The changes during the drying were microstructurally examined using Scanning Electron Microscope (a JEOL model JSM 5800LV) at 5 kV. The samples were coated with gold in a Sputter Coater (BALZERS, model SCD050).

Statistical analysis
Each experiment was conducted in triplicate. Statistical analysis was performed using Origin Ro 2018, and ± SD was determined by Tukey's tests at p < 0.05.

Drying kinetics curve
The drying kinetic curves of selenium-enriched germinated black rice (SeGBR) at various ultrasonic times and drying temperature is depicted in Fig. 1. The moisture content of germinated rice decreased rapidly during the first 20 min of drying, and the rate of moisture loss was the same for all treatments. This might be because the moisture in the samples at this early stage was mostly on the surface of the samples, where it could quickly vaporize and cause similar drying rates regardless of the sample type [21]. However, after this period, there was a significant difference in the moisture, with SeGBR treated in the US (20 min) exhibiting a lower moisture content than the US at 10 and 20 min in control samples at all drying temperatures. The US (20 min) ultrasonically treated group had the fastest sample drying time at 50 • C (p < 0.05) than other US treated and control sample groups. Ultrasound may induce rapid expansion and contraction of plant cells, which could develop air bubbles in and around the sample. As a result, sizeable transient pressure variations cause changes in plant cell viscosity and surface tension, as well as the formation of microchannels, thereby decreasing the drying time [9]. The findings of this study are comparable to the drying kinetics of germinated barley seeds dried by ultrasonic-assisted hot-air drying [3]. The drying temperature of SeGBR is likewise affected by ultrasonic time. As shown in Fig. 1, when the ultrasonic duration is 20 min, and the drying temperature is 50, 60, or 70 • C, the ultrasonic group requires 110, 130, or 150 min of drying time, respectively.
In comparison to the control group, drying time was decreased by 20.5%. Thus, ultrasound in the drying process can speed up the moisture distribution within the sample, thereby reducing the drying time and increasing the drying rate. The ultrasound waves produced by the radiation disk can penetrate directly into the material when the sample is placed on an ultrasonic radiation disk (HAD). Ultrasound has high frequency vibrations that dramatically and repeatedly stretch and contract the microstructure of samples, creating many microbubbles within the materials. Blasting those bubbles instantly could generate kinetic and compressive energy [22].

Drying rate curves
The changes in drying rate versus drying time are depicted in Fig. 2. The drying rate decreased as the process progressed under various drying temperatures and ultrasound timings, and no constant rate period was noticed. At 10-, 20-, and 50-min US timing, the treatment groups had 9.45, 21.5, and 28.5% higher initial drying rates than the control group. In contrast, the 20-min ultrasound group at 60 • C and 70 • C dried faster. This could be because germinated black rice experiences an enormous heat transfer homogenous mass drive when subjected to high ultrasonic power (500 W), which causes significant internal heat production due to the high energy absorption of the grain layer (GBR) [3]. However, drying rates vary significantly at different temperatures due to an imbalance in the vapor pressure differential between the grain's center and surface during drying, which causes unstable moisture transport from the inner core to its surface [23]. The results showed that increasing ultrasonic intensity in hot-air drying led to a higher SeGBR drying rate. Applying ultrasonic waves to a solid/gas system causes variations in oscillation speed, microfluidics, and pressure at the interface, resulting in mechanical agitation of the gas medium. This agitation facilitates the transfer of water from the sample surface to the air, thus accelerating the drying process [24]. Fig. 2b shows that all processed GBR drying rates started with a longer drying time and then gradually decreased. Similar drying rates have been found for microwave drying of apple pomace [25], apple slices [26], and grapes [27], demonstrating that the warm-up time is minimal and the decrease rate is relatively lengthy. The rapid drying rate at the start of drying might be ascribed to the vast quantity of free water inside the GBR grain and the significant ultrasonic absorption capacity, resulting in a massive driving force for water transfer. Furthermore, once the maximum drying rate is achieved, the drying process fully enters all treatment rate decrease phases. It demonstrates that the SeGBR hot-air drying process may be regarded as a falling rate owing to a rapid warm-up rate during the drying phase. In general, the drop rate period dominates the drying process for most fruits and vegetables, where moisture transport is limited by internal heat and mass transfer resistance [28,29].

Mathematical modeling of SeGBR
The drying kinetics of selenium-enriched germinated black rice during hot-air drying were described using drying models. The results of fitting the specified mathematical layer model to the experimental moisture data are depicted in Table 1. Mathematical models can help us predict the drying effect through statistical analysis. Even though all the selected 15 models fitted satisfactorily with the experimental data, the Hii model is said to effectively describe the drying kinetics of GBR with the highest R 2 (>0.997 to 1.00), lowest RMSE (<0.0027 to 0.0062) and lowest RSS (<0.00) for all drying conditions (Table 1). Fig. 3 displays the efficacy of the Hii model via a comparison of the predicted humidity data with the experimental data. Therefore, the Hii model was used to estimate moisture rate data under various drying settings, followed by linear regression, demonstrating the model's adaptability to represent the hot-air drying behavior of black rice. According to Le & Jittanit [30], the Hii model predicts jasmine brown rice's drying kinetics well. Similar results have been reported about using Hii models to explain sweet potatoes' drying performance [31]. In addition to providing relatively accurate quantifications, model fitting could be used to compare drying kinetics under different conditions, providing detailed information about drying time, rate, sensitivity, and resistance to different treatments during the whole drying process [32].

Activation energy (Ea)
The activation energy (Ea) is crucial in determining the minimum energy required for moisture diffusion within a material. The results of the activation energy in the untreated and ultrasonic-treated SeGBR drying process are shown in Table 2. The activation energy increased Table 1 Averages of selected models fitted to the ultrasound-assisted hot-air drying for selenium-enriched germinated black rice.  from 3.97 kJ/mol to 13.90 kJ/mol for samples treated with ultrasound for 10, 20, and 30 min, respectively. Comparing the activation energies of ultrasound-treated and untreated samples revealed an increase, whereas the control group requires significantly more energy, 20 Kh/ mol, to initiate the drying process, as shown in Fig. 1 and Fig. 2. This increase suggests that ultrasound-treated SeGBR needs more energy to start diffusion. The temperature sensitivity of the process could account for the rise in activation energy, indicating that sonicated samples are more prone to diffusion than controls. The activation energy of agricultural products varies, ranging from 12.7 to 110 kJ/mol [33]. Rashid et al. [34] discovered a similar range of 11.03 to 57.42 kJ/mol while examining the effects of various multi-frequency ultrasonic waves on the infrared drying of sweet potatoes. The activation energy for drying wheat increased as the heating rate increased. At a heating rate of 2 C/ min, the activation energy was 14.76 kJ/mol, while it rose to 28.17 kJ/ mol at a heating rate of 10 C/min [35]. Although the ultrasound is much shorter than we used, the activation energy values are comparable to our work.

Thermodynamics properties
The thermodynamic properties studied in this investigation were primarily linked to forming an intermediate product, also known as a transition state or activated complex. The thermodynamics of drying can be fully explained by the thermo-kinetics of drying, where specific entropy is related to the amount of energy needed to remove the water absorbed by the product [36,37]. The drying process's specific enthalpy (H) reduced marginally in both untreated and ultrasound-treated SeGBR samples. However, raising the drying temperature led to a decrease in enthalpy. The enthalpy values recorded were positive, in line with the findings of Rashid et al. [34] in their study of drying sweet potatoes. This study demonstrates that an endothermic reaction occurs when the activated complex (PD -H2O) is produced, proving that the physical and chemical conversion needs heat [38,39]. According to Al-Zybaidy & Khalil [40], ΔH indicates the difference between the active state and the reactant; hence it must be positive. The endothermic reaction in this study suggests that the addition of energy at drying temperature or ultrasound may have aided activation complex formation. The study found that the energy required for the reaction (enthalpy of activation) in samples treated with sonication was lower than in untreated black rice. This suggests that ultrasound combined with different temperatures can supply the necessary energy to form the activated complex, resulting in faster moisture absorption, as demonstrated in Figs. 1 and 2.
In contrast, the value of entropy (S) in untreated and sonicated selenium-rich germinated black rice is negative. Entropy is often described as the degree of the disorder [41]. Our study showed that the modulus entropy of sonicated samples was greater than that of untreated samples. This suggests an increase in the system order, which is against entropy's nature of leading to disorder. The molecular structure, in this instance, became more organized [39]. H-bonds formed between water and other substances are believed to be more structured during hydration than H-bonds within the water. Ultrasound measurements from US-RBSe1 to US-RBSe9 showed higher entropy values compared to control samples Con1 to Con3. This could be because the ultrasonic technology causes chaotic and turbulent movement in the molecules, hindering the formation of an organized molecular structure during activation. Therefore, a more significant reduction in entropy was required for the process to form the activated complex to achieve this organization. The organization faces difficulties with ultrasound processing as the sonolysis produced by cavitation creates free radicals in water. A similar effect was observed during rice parboiling, showing that entropy was negative and constant with temperature, indicating no significant increase in disorder in the system [42]. Jideani & Mpotokwana [43] found that the entropy of Botswana Bambara groundnut varieties decreased during hydration, demonstrating an increase in order within the system, resulting in a less random arrangement. In addition, the drying temperature increase led to a rise in the process entropy. Rashid et al. [34] also reported this temperature effect for sweet potatoes.
The Gibbs free energy (ΔG) of untreated and ultrasonic-treated black rice is positive, indicating that the reaction is not spontaneous. Temperature and ultrasonic levels have the most negligible impact. The drying temperature causes an increase in Gibbs free energy, which is identical to the rise in ultrasonic-dried samples at 50, 60, and 90 • C. The outcomes are comparable to those of Miao et al. [38] regarding common beans. Borges et al. [44] state that the ΔG value measures the work that promotes grain moisture. A positive Gibbs free energy value implies that the drying process is exothermic, needing heat from the environment. However, the rise due to ultrasonic processing shows that less heat is necessary for water breakdown. Nadi & Tzempelikos [39] found similar results during the vacuum drying of apples, and Souza et al. [45] observed the same trend in the dehydration of mesocarp in pequi or souari nut (Caryocar brasiliense). This study's results show that the black rice drying process is not spontaneous and requires additional energy from the environment.
Furthermore, the high molecular organization was noted during this transition. Miano et al. [38] also found that the thermodynamic properties of the activated complex exhibit low-temperature dependence, which was confirmed in our research. Our findings indicate that increasing the temperature does not influence the spontaneous formation of the activated complex, the degree of molecular organization or the energies involved.

Specific energy consumptions (SEC)
The SEC was established as the total energy required to reduce the moisture content of germinated black rice to 10 ± 0.5 kg of water per kg Table 2 Effect of ultrasound-assisted hot-air drying on selenium-enriched germinated black rice regarding activation energy, thermodynamics, and specific energy consumption. values. An increase in the temperature difference between the ambient air and the drying air, as well as a decrease in the hot-air blower's power consumption, likely led to the observed result [46]. High air temperatures create more water vapor pressure within the kernels, leading to the release of moisture. However, this increase in energy usage also produces a higher amount of water vapor during the drying process. As a result, the diffusion coefficient grows, and the effective conductance (EC) falls [47]. Similar outcomes have been observed for the air ultrasonic drying of corn husks when the energy required for drying air exceeds the energy saved due to the reduction in drying time [48]. In addition, ultrasonic times of 20 and 50 min can shorten the total drying time, reducing energy usage. According to the results above, an increase in ultrasonic processing time can achieve products with a high rehydration ratio and an excellent appearance. However, high ultrasound power (500 kW) can result in increased ultrasound energy loss and adverse tissue damage; therefore, ultrasound with 500 kW high power and longer ultrasound timing (20 & 50 min) is suitable for accelerating the SeGBR drying process. Higher drying temperatures result in shorter drying times, lower operating costs, and better quality, but they can also cause higher nutrient degradation rates, resulting in inappropriate damage. As a result, a drying temperature of 50-70 • C is appropriate for achieving an increased drying rate while maintaining product quality. Similar results were obtained in rough rice using convection drying assisted by air-borne ultrasound [49].

Individual phenolics compounds
The contents of individual phenolic compounds in the seleniumenriched germinated rice acquired from black rice and control samples were determined using HPLC-PDA. Due to the lack of available standards, only 11 phenolic and 3 flavonoid compounds and 6 anthocyanidins were quantified.
The phenolic acid profile of SeGBR was found to contain high levels of ellagic acid, gallic acid, Isoferulic acid, protocatechuic acid, and ellagic acid. Among these compounds, gallic acid was the most abundant in all samples of black rice. This result aligns with previous studies conducted by [50]. While gallic and ellagic acid were reported as the most abundant phenolic compounds by Hrnčič et al. [51]. As shown in Table-3, the US treatment significantly increased the extractability of almost all phenolic compounds compared to the control, while the concentrations of ferulic acid, p-coumaric acid, glucosic acid, and vanillic acid remained unchanged. Therefore, the ultrasound may induce the fragmentation of conjugated phenolic acids into accessible form, resulting in increased extractability of the samples examined here. High polyphenols extraction yields can be related to US-induced cell membrane rupture, which facilitates phenolics extraction from plant cells [52]. The highest concentration of gallic acid found in the US-SeGBR4 ultrasound-treated sample was 2605.94 µg/g, followed by 1455.15 µg/g in US-SeGBR6. Protocatechuic acid showed the second highest amount, ranging from 908.42 to 115.22 µg/g. The drying process significantly impacted the levels of individual phenolic compounds. The effects on the metabolomics of peanut kernels were explored by Talcott et al. [53], who discovered substantial changes in the levels of amino acids, organic acids, and sugars, as well as higher amounts of various phenolic compounds. Some phenolic components, such as ferulic acid, hydroxybenzoic acid, p-coumaric acid, and vanillic acid treated with sonication, were significantly lower in all drying treatments compared to control samples. However, the amounts of phenolic compounds such as ferulic acid in US-SeGBR1, US-SeGBR5, US-SeGBR7, and US-SeGBR8 samples were not identified in ultrasonic treated dried samples. The study established the following order of total phenolic acids in the ultrasound-treated samples of black rice: US-SeGBR4 (4554.75 µg/g) > US-SeGBR6 (2751.43 µg/g) > US-SeGBR3 (2471.27 µg/g) > US-SeGBR2 (2377.99 µg/g) > US-SeGBR1 (2193.92 µg/g) > US-SeGBR7 (1404.82 µg/g) > US-SeGBR5 (1380.23 µg/g) > US-SeGBR7 (1117.96 µg/g). Drying at higher temperatures is more likely to result in the loss of conjugated polyphenolic compounds, as evidenced by the fact that the content of phenolic acid is highest at 50 • C, lower at 60 • C, and highest at 70 • C. Organic edible seed germination and sprouting results in nutritional sprout products that are good alternatives to traditional plant foods. Recent research has confirmed that energy metabolism plays an important regulatory function in the postharvest preservation of fruits and vegetables. Previous research demonstrated that a positive energy status was critical for improving the production of nutritional chemicals such as phenolics [54]. Similar results were obtained in the germination of barley seeds using ultrasound-assisted drying [3].
Kaempferol was the most prevalent flavonoid, followed by quercetin; rutin was present in all samples but at low concentrations (both control and ultrasound-treated). These flavonoid compounds in black rice husks are advantageous because they have antipyretic, analgesic, antiinflammatory, antiarthritic, antioxidant, and immunomodulatory activities [1]. The highest concentrations of kaempferol (62.39 g/g) and quercetin (49.88 g/g) were found in the US-SeGBR7 and US-SeGBR8 samples, respectively. Kaempferol is typically less prevalent than gallic acid in most products [50]. There was a marginal gain in sonicated samples relative to flavanol controls. The increase could be attributed partly to the increased extractability of flavanols in dry soft tissues and the release of phenolic compounds bound to the cell wall [55]. In addition, the quercetin content of all treatments extracted with methanol is higher than that of those removed with water (22.1 g/g) [56]. The same results were found when comparing the number of flavanols with the ultrasonic-assisted extraction of phenols from Chinese wild rice [57].

Volatile profiling of Se-GBR
The flavor and aroma of rice significantly impact the sensory characteristics of cereals [64]. GC-MS analysis of 12 samples extracted using SPME fibers and analyzed using MassLynx software identified >50 volatile compounds, excluding those that were untraceable or impure (Table 4). A total of 55 volatiles were identified, including 10 classes such as 4 alcohols, 1 acid, 6 aldehydes, 12 alkane, 4 alkenes, 7 alkaloids, 4 ester, 9 ketones, 6 phenol, and 2 pyrazines. Among these compounds, aldehydes were quantitatively the major groups detected only in ultrasound-treated samples, whereas alkane, ketone, and phenol were the second significant groups detected in both control and ultrasoundtreated samples [65].
The establishment of lipid secondary metabolites during linoleic acid degradation resulted in the formation of volatile aldehydes and alkanes via autooxidation and/or photosensitive oxidation of fatty acid esters hydroperoxide to form alkoxy radicals [6]. According to a study by [66], aldehydes and alkanes have been identified as crucial contributors to the aroma of rice due to their low odor threshold. These compounds are believed to be the primary source of barley and white wheat flour aroma. The results from the sample analysis could be used to determine the level of lipid oxidation and assess the quality of rice during storage. All of the ultrasound-treated samples contained alkanes dodecane and tetradecane. In addition, most compounds across 10 classes were identified in the ultrasound-treated samples compared to the control, indicating that ultrasound better preserves the volatile compounds at drying temperatures of 50, 60, and 70 • C. Similar research reportedly demonstrates that ultrasound pretreatment significantly impacts volatile components, aldehydes, and ketones, the essential aromatic compounds [67]. The proportion of aldehydes, alkanes, ketones, and phenols in every sample exceeded 75%. Compared to the untreated sample, the total content of compounds increased immediately after ultrasonic treatment for 10 min and 20 min, and the value increased as the ultrasonic time increased. The rise in compounds in ultrasonic SeGBR agrees with previous observations of germinated whole brown rice [68]. This is believed to result from the destruction of cell structures that leads to the release of free fatty acids (FFA) and the subsequent interactions between FFA and macromolecules. The variations observed in the total amount of identified volatile components may be linked to negative impacts related to lipid oxidation and FFA formation in response to various treatments [69]. However, there was a statistically significant difference (P > 0.05) in the drying of the samples after sonication. Despite being the most abundant classes, none of the acids are known to be associated with rice flavor. Fig. 5a & 5b show the results of a detailed analysis of the volatile components of samples subjected to varying degrees of ultrasound and drying, suggesting that the effects of these processes on the volatile composition were qualitatively similar but quantitatively different for specific components. According to Table 4, the compound most impacted by drying is butyric acid, 1-propyl pentyl ester in the acid, which is found in US-SeGBR7. This is primarily due to glucosinolates (GLS) and their breakdown products, such as isothiocyanates, organic cyanides, oxazolidine thione, and thiocyanates, which give the rice its characteristic odor and flavor [70]. Compared to untreated samples, the concentrations of alcohols, acids, aldehydes, ketones, and other compounds in untreated samples are substantially lower and much lower. According to the findings, the ultrasound process significantly impacted the volatile compounds of selenium-enriched black rice, and the most abundant butyric acid compound, 1-propyl pentyl ester in acids in black rice, was also the most affected compound. Untreated samples showed a decrease in volatiles due to biotransformation and diffusion during steeping, aligning with our previously reported results [68]. A study on the effects of ultrasonic treatment on the volatile compounds of roasted coffee beans discovered that ultrasonic treatment increased the concentration of several volatile compounds, including pyrazines and furans, while decreasing the levels of others, such as 2,3-pentanedione and 2-methyl pyrazine [71]. Another study on the effects of ultrasonic Table 4 Effect of ultrasound-assisted hot-air drying on selenium-enriched germinated black rice regarding volatile compounds. treatment on the volatile profile of Shanlan rice discovered that ultrasonic treatment resulted in significant changes in the concentration and composition of several volatile compounds, including an increase in the levels of ethyl hexanoate and a decrease in the levels of other volatile compounds like ethyl caprylate [72]. The results here reveal that the volatile compounds in dried black rice undergo distinct changes depending on the ultrasonic treatment and drying method. In order to better understand the effects of ultrasound and drying technology on the volatile compounds of selenium-rich germinated black rice, more research is needed to clarify the mechanism and pathway of volatiles derivation under techniques.

Microstructural changes
The microstructure of dried selenium-rich black rice, both untreated and after ultrasonic treatment, has been shown to change significantly ( Fig. 6a− 6l). The microstructure modification and starch gelatinization properties of ultrasonically-dried SeGBR were analyzed to understand better the factors that contributed to the observed changes in drying characteristics. Water diffusion and evaporation in SeGBR may be related to microstructure changes caused by ultrasound and hot-air drying. Fig. 6a-c demonstrates the compact structure and regular arrangement of starch particles in untreated SeGBR. Starch granules' tightly packed system is slightly broken up during thermal drying. According to a study by Shen et al. [73], in the initial drying stage, intense moisture diffusion within SeGBR caused the microscopic pores in the starch structure to collapse due to irradiation. This collapse facilitated the process of moisture diffusion and evaporation, leading to an increased drying rate. The surfaces of the samples that were sonicated for 10 min at 50, 60, and 70 • C exhibit gaps and deformations, notably along the cell walls of adjacent cells, as depicted in Fig. 6d-f. The surfaces of the rice samples sonicated for 20 min in Fig. 6g-i was nearly identical. The formation of these pores and cracks on the microscale (µm scale) is related to cavitation bubbles and results in microscale physical activities like shock waves, moisture jets impinging on solid-liquid interfaces at speeds of up to 200 m/s, and high-stress rates triggered by high-frequency heating/cooling rates [74]. According to reports, the collapse and cavitation bubbles that occur on or near the plant surface cause micro-cracks on the thin soybean flakes [5]. The alterations in surface structure would also increase mass transfer. Fig. 6j-1 shows that ultrasonic treatment of black rice for 50 min also altered its original form. There were also visible fissures on the outermost layer. These alterations in rice's microstructure may be explained by the cavitation effect produced by ultrasonic and enzymatic breakdown of cellulose. As seen in Figs. 1 & 2, these effects improve water absorption during drying, reducing drying time.
The modification of the surface morphology also results in improved mass transfer. For instance, when ultrasound was applied to mung beans, hydration time was decreased by 25% Miano et al. [75], while it lowered hydration time in maize kernels by 35%. [76]. In the present research, the microscopic pores on the surface of germinated rice grains give a new path for water to enter the rice grains. As a result, ultrasonic pretreatment of SeGBR is predicted to improve hydration and minimize drying time. Furthermore, ultrasonic stimulation accelerated the pattern of morphological change, as evidenced by starch erosion over untreated samples with higher prominence and lower starch diameter. The findings indicated that ultrasonic pretreatment might increase starch granule disintegration during germination.

Effect of ultrasound and drying on Se content
Significant differences (p < 0.05) were found in the selenium concentration of the germinated black rice samples (Fig. 7). Changes in the selenium concentration of various samples may result from the absorption of selenium solution throughout the 60-h germination period. Seeds absorb selenite to produce selenides through the catalysis of glutathione. When catalyzed by amino acid synthetases, Selenides generate selenium-containing amino acids, including SeCys and SeMet.
The selenium concentration of black rice was increased in US-SeGBR4 and US-SeGBR1 but decreased in Con3 at 70 • C. The findings are consistent with the previously reported range of brown rice selenium levels, 2.39 lg/100 g [4]. As indicated in Fig. 7, the selenium content of black rice varied considerably with various treatments (p < 0.05). A decrease in temperature was seen between the control sample and the ultrasonic treatment sample, which ranged in temperature from 50 • C to 70 • C. However, the value of selenium in the samples treated with ultrasound was somewhat more significant than in the control group. High levels of ultrasonic energy can increase the amount of organic selenium produced. This happens because ultrasound disrupts the tissue structure of plants, making it easier to extract phytochemicals and thereby raising the content of selenium [77]. An investigation into the effects of ultrasonic treatment on selenium bioavailability in wheat discovered that ultrasonic treatment significantly improved selenium bioavailability through promoting the release of bound selenium and altering it into more bioavailable forms [78]. Another investigation on the effects on selenium bioavailability in soybean sprouts discovered that selenium biofortification greatly enhanced total selenium content and bioavailability of selenium in the sprouts [79].
The organic selenium level of black rice was similarly impacted by drying temperature. Lower drying temperatures, such as 50 • C, have greater selenium concentration than 60 and 70 • C. When the drying temperatures were set to 50 • C, the selenium concentration of the Us-SeGBR4 (10 min) sonicated sample was 1.05 times greater than that of the control samples. Higher drying temperatures may inhibit amino acid synthase activity, resulting in less selenium generated during drying; hence, lower drying temperatures are more likely to increase selenium retention [3]. Reyes et al. [80] concluded that drying reduced broccoli's total selenium concentration by 35% compared to fresh selenium-rich veggies.

Conclusion
This study investigated the changes in the main quality features of ultrasonically treated selenium-enhanced germinated black rice throughout the hot-air drying process for the first time. In contrast to the control and US-SeGBR pretreatments, substantial differences in drying kinetics, moisture loss, selenium impact, and quality metrics were observed throughout the drying process of germinated black rice. Compared to traditional hot-air drying, the use of ultrasound in conjunction with hot air can result in a significant improvement in drying speed and a reduction in the drying time. The increased ultrasonic timings and drying temperatures enhanced the effectiveness. However, the ultrasound's strengthening effects were diminished by the decrease in the water content of SeGBR after drying. The ultrasoundtreated group required a minimal activation energy of 3.97 kJ/mol to begin the drying process after 20 min, demonstrating the usefulness of ultrasound in the drying process. Although all fifteen models matched the experimental data sufficiently, the Hii model suited the drying kinetics of SeGBR the best, with the greatest R 2 (>0.997 to 1.00), lowest RMSE (0.0027 to 0.0062), and lowest RSS (0.00) for all drying settings.
It was discovered that phenolic profile and anthocyanins content might be raised with ultrasound-enhanced hot-air drying. The volatile compounds profile was significantly enhanced in the sonicated samples using HS-SPME and GC-MS analyses. The key benefit of using ultrasound in conjunction with hot-air drying for preserving the quality of selenium-enriched black garlic (SeGBR) is the preservation of organic selenium. SEM micrographs taken during the process confirmed the presence of micro cavitation in the grains, contributing to increased mass transfer. Based on these results, it can be concluded that integrating ultrasonic technology into the drying process can significantly speed it up while maintaining the quality of the dried black rice.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.